1. Field of the Invention
The invention relates to a lock-up clutch for a hydrodynamic torque converter which comprises a piston connectable to the converter housing through at least one friction lining and deflectable in the axial direction, wherein the interface between the friction lining and at least one of the converter housing and piston allows the passage of hydraulic fluid from the converter circuit for component cooling.
2. Description of the Prior Art
A prior art hydrodynamic torque converter with a lock-up clutch is disclosed in German reference DE 44 23 640 A1. FIG. 3 of that reference shows a piston connectable to the converter housing through a friction lining and capable of a deflection movement in the axial direction. Other Figs. of the same document indicate that a depression for the passage of hydraulic fluid from the converter circuit may be arranged on the friction lining as well as in the one or more converter component(s) interacting therewith, such as a converter housing. Associated with the depression is an inflow or outflow, the former opening radially outside into the converter circuit and the latter opening radially inside into a chamber located between the converter housing and the piston.
In one embodiment shown in the prior art reference, the depressions run radially from the outside to the inside, so that hydraulic fluid flows very quickly through these depressions and creates a correspondingly low heat exchange with the converter components accommodating them. The depressions may also be very complex in form. In the latter case, the depressions can be manufactured only at high cost and material expenditure and are thus basically ruled out for series converters, in which simplicity and low cost are of essential significance.
One method of removing material from the area of the subsequent depression is stamping, for example. According to present-day stamping techniques, the friction lining is divided into individual segments. As result, during the subsequent assembly of these segments and their insertion onto the piston, inaccurate positioning of the segments is possible and may result in an unwanted change in the depression geometry.
This problem does not exist when an embossing process is used. However, the depth of the depressions that are manufactured using this method is relatively slight. Deviations from the target depth have considerable influence on the flow amount and thus on the heat extraction capacity.
Regardless of whether the depressions are manufactured by embossing or stamping, a setting of the lining occurs during operation, reducing the depth of the depressions. As a result, the flow resistance changes, so that, in turn, the requisite cooling is not guaranteed.
According to another prior art reference, DE 44 32 624 C1, depressions are embodied in the friction lining, whereby a zig-zag-shaped through channel for hydraulic fluid runs circumferentially and is associated with inflow or outflow depressions, which are located at predeterminable distances in the circumferential direction. Although a friction lining of this type may have desirable cooling properties, it is often impossible, for reasons of production technology, to precisely maintain the geometry and/or depth of its depressions. As a result, unwanted fluctuations occur in the volume flow passing through the depressions, so that the maximum friction surface temperature demanded by the customer cannot be guaranteed with sufficient reliability. If the maximum friction surface temperatures are exceeded, the friction lining may be destroyed by overheating.
Another prior art device is disclosed in EP 0 407 895 A2 that includes depressions for the passage of hydraulic fluid in the extension area of a friction lining. In some embodiments, these depressions are provided in the friction linings; in others, in the piston of the lock-up clutch. The depressions run around the circumference in circular fashion and are interrupted by radial depressions arranged at predeterminable angular distances from each other. A portion of these depressions acts as the outflow for hydraulic fluid and another portion acts as the inflow.
In the aforementioned prior art documents, the inflow and outflow depressions as well as the passageway depressions are always embodied on the same component, such, for example, as the friction lining or the piston of a lock-up clutch. For this reason, the production of these depressions is also linked to the problem of imprecise manufacture.
A further embodiment of a depression in a converter component is disclosed in FIG. 9b of U.S. Pat. No. 5,248,016. In this device, flow-regulating holes are provided in the piston of a lock-up clutch on the side of the piston that faces the converter circuit. These flow-regulating holes pass into channels embodied on the piston on the side of the piston that faces a friction lining.
In this embodiment of depressions, the flow-through amount is determined by the diameter of the flow-regulating opening, not by technical manufacturing inaccuracies in the depression. However, the depressions disclosed in the aforementioned patent are either embodied in such a way that hydraulic fluid flows through them too quickly for adequate heat exchange, or else are very expensive with respect to design.